This disclosed device and method relates generally to a transfer station used in electrostatographic or xerographic printing.
The basic process steps of electrostatographic printing, such as xerography or iconography include creating an image with the toner particles which is transferred to a print medium, which is typically a sheet of paper but which could be any kind of substrate, including an intermediate transfer belt or continuous web. This transfer is typically carried out by the creation of a “transfer zone” of electric fields where the print sheet is in contact with, or otherwise proximate to, the photoreceptor. Devices to create this transfer zone are well known in the prior art.
For example, the use of BTR (Biased Transfer Roll) foam rollers to either pull an image from a PR (Photoreceptor) belt or drum to an intermediate belt or from an intermediate belt to paper are often used. Typically, in such transfer operations, as shown in U.S. Pat. Nos. 7,242,894 and 7,158,746, a biased transfer roll is disposed in contact with a portion of a photoreceptor, thus forming an image transfer nip. An image-receiving sheet passes through the nip between the photoreceptor and transfer roll. At the nip itself, a toner image on the photoreceptor is transferred to the sheet by a combination of physical pressure at the nip, caused at least in part by the transfer roll, and an electrical bias placed on the transfer roll by suitable circuitry.
However, if the paper or substrate being fed is not a cut sheet, but rather a continuous roll of sheet paper or label media, the standard transfer process is inadequate. The conversion of a high speed, high volume Xerographic machine with a cut sheet paper supply to a continuous paper roll feed for label or book production requires an entirely new transfer area, that will not disturb the unfused toner either by lateral or process direction shear forces resulting from velocity mis-matches or from air breakdown while the media makes contact to the belt. Various events must be considered such as the skipping of the photoreceptor (PR) belt seam and skipping various other images on the belt such as test patches, in order that the pitch to pitch distance of images transferred to the paper and paper roll feed is held consistent. The new system must also be configured such that air break down does not occur disturbing the image by reducing the nip area, pre-wrapping the PR assist roll, and sufficient attack exit angle.
In web feeding, the image substrate material is typically fed from large rolls of paper in a defined width as previously stated. A difficulty, however, in printing from an endless belt type photoreceptor printing engine onto a continuous web substrate is the fact that belt type photoreceptors typically have a belt seam where the two ends of the belt are fastened to one another to form a continuous loop. Typically, it is either impossible or undesirable to form images overlying this belt seam, resulting in asynchronous or irregularly spaced image production. This, in general, can be a significant problem to the transfer of those images to a substrate. The problem is more severe, in particular, in the synchronization of images with a continuous web substrate.
Heretofore, it has been difficult or impractical to rapidly start and stop paper webs running through a printing system at high speeds because of the danger of web tearing, slippage, or misregistration, and/or the large moment and mass of the paper roll. As disclosed in U.S. Pat. No. 5,970,304, buffer loops and dancer rolls are known for the buffering of web speed variations and also the separation of the web from the nip to adjust the relationship of the photoreceptor belt and web for facilitating the transfer of images from the belt to the web.
It would also be desirable to provide other possible advantages to prior continuous paper feed systems such as better registration error control, and a smaller transfer nip. For example, a BTR transfer zone is typically only 3-5 mm, which makes it easier to insure good image quality and low shear area due to either web velocity mis-match errors or lateral position error moments. Also, it may be desirable to fully strip the web with the image prior to the seam before disengaging the web from the photoreceptor.
Thus, in order to maintain the continuous paper web feed pitch and compensate for occurrences such as the need to avoid the seam on the PR belt, a BTR roll is provided at the transfer zone or station and the paper web separated from the BTR nip. The continuous paper web is driven backwards and then accelerated to position the paper web at exactly the correct location prior to the paper web and PR belt uniting at the BTR roll nip. This is known as a ‘Pilgrim step’ in the converting industry.
In operation, according to the disclosure, a suitable BTR roll, often a soft foam roll, when engaged with an auxiliary or stripper roll will produce a nip of 3-5 mm wide for generating a transfer field and depositing a positive tacking charge to the backside of the paper. The toner is negative and is drawn to the paper from the photoreceptor belt. The coordination of web tension, auxiliary roll, and BTR roll will provide controllable belt engagement and defined timing of transfer of image without destructive uncontrolled air breakdown to the image. The timing of the auxiliary roll and BTR roll engagement after reversing will allow for synchronization of the turn on of the field in the gap between images without creating toner disturbances.